What is Hashmap?

In Java, a HashMap is a part of the java.util package and implements the Map interface. It stores data in key-value pairs, where each key is unique and maps to exactly one value.

🧠 Key Features of HashMap:

• Fast access: Uses a technique called hashing to retrieve values quickly based on their keys.
• No duplicate keys: If you try to insert a key that already exists, the new value will overwrite the old one.
• Allows one null key and multiple null values.
• Not ordered: The order of elements is not guaranteed (use LinkedHashMap if you need insertion order).
• Not thread-safe: For concurrent access, use ConcurrentHashMap.

🧪 Example:

import java.util.HashMap;

public class Example {
public static void main(String[] args) {
HashMap scores = new HashMap<>();
scores.put(“Alice”, 90);
scores.put(“Bob”, 85);
scores.put(“Charlie”, 92);

    System.out.println(scores.get("Alice")); // Output: 90
}

}Java’s HashMap class offers a rich set of methods to manage key-value pairs efficiently. Here’s a handy breakdown of the most commonly used ones:

🔑 Basic Operations

• put(K key, V value) – Adds or updates a key-value pair.
• get(Object key) – Retrieves the value for a given key.
• remove(Object key) – Deletes the entry for the specified key.
• clear() – Removes all entries from the map.
• size() – Returns the number of key-value pairs.
• isEmpty() – Checks if the map is empty.

🔍 Search & Check

• containsKey(Object key) – Checks if a key exists.
• containsValue(Object value) – Checks if a value exists.
• getOrDefault(Object key, V defaultValue) – Returns the value or a default if the key isn’t found.

🛠️ Advanced Utilities

• putIfAbsent(K key, V value) – Adds a key-value pair only if the key isn’t already present.
• replace(K key, V value) – Replaces the value for a key if it exists.
• replace(K key, V oldValue, V newValue) – Replaces only if the current value matches the old one.
• merge(K key, V value, BiFunction) – Combines values for duplicate keys using a function.
• compute(K key, BiFunction) – Recomputes the value for a key.
• computeIfAbsent(K key, Function) – Computes and adds a value if the key is missing.
• computeIfPresent(K key, BiFunction) – Updates the value only if the key exists.

🔄 Iteration & Views

• keySet() – Returns a Set of all keys.
• values() – Returns a Collection of all values.
• entrySet() – Returns a Set of all key-value pairs.
• forEach(BiConsumer) – Performs an action for each entry.